All, I put a nice chunk of native Cumberland valley limestone in my downspout drains. It helps to buffer the acid rain, slows the flow to allow more time to soak into the ground, and helps to keep the drainpipe stationary. To wit:

I mean this as no disrespect and I like the thought. But your probably not getting much buffering with that large of a piece of limestone. You would be much better off with a bed of fine gravel that increases the retention time of the water. Also the surface area is greatly increased to allow more water in contact with the limestone.

Posted on: 2014/4/17 12:19

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"Distrust any enterprise that requires new clothes"-Henry David Thoreau

I was waiting for that very accurate observation to be made. Larger stone did not work for very long in creeks because of the surface film that developed on the rocks in short order, sealing the calcium carbonate from exposure to the acidic water. Smaller stones provide more surface area too. Limestone sand is even better. JG's heart is in the right place though.

You could whack your downspout up a couple feet and have it spill into a large flower pot with holes in the bottom and filled with fine limestone gravel. This will give you good retention time and be easy to maintain. Just stir up the stone periodically and hose out with clean water. That should knock off any mineral armoring that has taken place and flush out the dirt and debris.

The smaller the stone the better buffering you will get due to increased surface area in contact with the water and increased retention time. Just remember that the finer the stone, the more it will need maintained due to the pore spaces being clogged by dirt, debris and mineral precipitation more readily.

Posted on: 2014/4/17 22:00

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"Distrust any enterprise that requires new clothes"-Henry David Thoreau

The most important thing with water flowing off roofs and paved surfaces is to get the water to infiltrate down into the groundwater, rather than running down the streets, into storm drains and flowing directly to the creeks and increasing the flashiness of the flow, and reducing the baseflow from groundwater. There are techniques for doing this at homes and businesses.

It just holds the spout extensions in place and breaks up the flow enough to prevent erosion of the yard/flower beds. I have rain barrels on the larger roof area drains. As stated I'm in limestone country and realize the rocks aren't doing much other than anchoring the drains. If money weren't the issue, I'd take the Frank Sawyer approach and throw copious amounts of chalk dust on all waterways -